The mechanism of capacity fade of rechargeable alkaline manganese dioxide zinc cells

Recent experiments showed, that in contrast to traditional opinion, if the cathode was protected by anode limitation the capacity fade of Rechargeable Alkaline Manganese Dioxide Zinc (RAM(TM)) cells was not caused by the EMD cathode, but by the gelled zinc anode. The key is the electrolyte. The cath ode competition for the electrolyte and the increasing requirement of chemi cally formed ZnO for more electrolyte caused an electrolyte deficiency at t he front face of the anode and finally caused precipitation of zincate and passivation of zinc. The crust is a mixed material of precipitation arid pa ssivation products. The low solubility ZnO is formed by decomposition of el ectrochemically generated zincate ions [Zn(OH)(4)](2-) and also by recombin ation of zinc with oxygen during overcharge. The progressively thickened "c rust" at the front face of the anode increases the resistance, then finally causes the cell to fade. The crusting is a redistribution of active materi al and electrolyte between the front and rear of the cylindrical gelled zin c anode. More electrolyte and proper charging can delay such a "crusting" p henomenon. (C) 2000 Elsevier Science S.A. All rights reserved.